1. Field of the Invention
The present invention relates to a seat lock device and more particularly, to a seat back device which applies forward-folding torque to a seat back of a seat upon releasing a seat lock state.
2. Description of Related Art
In general, a vehicle seat includes a seat, a seat back and a head rest. JP-A-2006-248330 discloses a seat lock device which fixes the seat back of a rear seat of a vehicle so as to be able to be folded forward. When it is configured that the seat back is able to be folded forward, it is possible to move a baggage from a baggage room to a vehicle room or to use the baggage room as wide as possible. The seat lock device fixes the upper portion of the seat back to a vehicle body so as to be attached thereto or detached therefrom, thereby preventing the upper portion of the seat back from oscillating during a driving. Alternatively, a bottom portion of the seat back may be axially supported so as to rotate. JP-A-60-88638 discloses an example in which a spiral spring is mounted to the bottom portion of the seat back. When the spiral spring is used and then the seat lock device is released, it is possible to easily fold forward the seat back in a horizontal posture by a force built up in the spiral spring.
When the rear seat is heavy and strong, it is necessary for the spiral spring to build up a force enough for folding forward the seat back. This is because the seat back is rotated about one end as a base thereof in the state where the seat back is inclined at about 120 degree from the horizontal position to the rear side. When a force of the spring becomes strong, elastic force becomes strong, so that the seat back is abruptly folded forward.
When a motor for auxiliary assisting the rotation of the seat back is employed, and the seat back is rotated so as to be erected from the substantially horizontal posture, at the timing of application of a repulsion of the spiral spring, there is a fear that it is misjudged that human body or a baggage is interposed between the seat back and the vehicle body and the motor is driven to invert and the seat back is returned to its original position. This case is shown in FIG. 5. A solid line indicates a rotation cycle of a motor to which no load is applied and a dashed line indicates a rotation cycle at the time something is inserted in the seat back. This is an example in which it is erroneously detected that the repulsion force of the spiral spring corresponds to a case where something is pinched between the seat back and the vehicle body when a slope angle of the seat back, that is, a recliner reaches a position at about 30 degree.
When attempting to attach the spiral spring as a helper spring of a forward-folding operation to a rotation center, it is necessary to provide the spiral spring for generating torque large enough to endure a weight of the seat back. At this time, when a sufficient clearance between a vehicle trim and a seat is not ensured, a problem arises in that it is difficult to design a layout in which the spiral spring is attached to the vehicle without giving a foreign-object feeling to a passenger.